1. Field of the Invention
The present invention relates to a lithographic method.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
There is a continuing desire to be able to generate patterns with finer resolution. In general, shorter wavelength radiation may be used in order to achieve a finer resolution pattern.
One method for providing patterns with increased resolution is dual trench patterning where a first pattern of trenches in a hardmask is overlaid by a second interleaved pattern of trenches in the hardmask to provide a final pattern in the hardmask of higher resolution than either the first or second patterns, which is then transferred to a target layer.
FIGS. 1a to 1j show schematically a dual trench patterning method for providing a high resolution pattern. FIG. 1a shows in cross-section a portion of a silicon substrate 2 provided with a target layer 4 of polysilicon material. A hardmask layer 6 is provided on the target layer 4. A first layer of bottom anti-reflective coating (BARC) material 8 is provided on the hardmask layer 6 and a first layer of photoresist 10 is provided on the first layer of BARC material 8. The hardmask layer 6 is typically formed from an oxide material, such as SiO2 or SiON. The photoresist 10 may be any appropriate type of photoresist, such as, but not limited to, positive tone photoresist.
A lithographic apparatus, which may for example be of the type shown schematically in FIG. 3, is used to expose a pattern in the photoresist 10. The exposed photoresist 10 is then removed using a developer, such as a caustic solution containing hydroxide ions, so that only unexposed photoresist 10 remains. The pattern is then transferred to the BARC material 8 using an appropriate etching process, such as an ion etch.
The resulting structure, shown in FIG. 1b, comprises four lines 12 which extend perpendicularly to the plane of FIG. 1b. The full width of each line 12 is three times the width of the space 14 between each pair of lines 12. Only four lines 12 are shown, but it will be appreciated that FIG. 1b shows only a portion of the substrate 2, and that many more lines 12 may be provided on the substrate 2.
Referring to FIG. 1c, the pattern formed in the photoresist 10 and BARC layer 8 is transferred to the hardmask layer 6 using, for example, a conventional hardmask etching process, such as an ion etch process. Referring to FIG. 1d, the first patterned BARC layer 8 and photoresist layer 10 are removed and then second layers of BARC 8a and photoresist 10a are provided on the patterned hardmask layer 6 as shown in FIG. 1e. A different pattern is then formed in the second layers of BARC 8a and photoresist 10a such that portions of the existing patterned hardmask 6 are uncovered, as shown in FIG. 1f. These uncovered portions of the hardmask 6 are then removed by a conventional etch process to yield the structure shown in FIG. 1g. 
With reference to FIG. 1h, the second BARC 8a and photoresist 10a layers are then removed using a conventional method to leave just the patterned hardmask 6 atop the polysilicon target layer 4. As shown in FIG. 1i, further etching (for example reactive ion etching (RIE)) is used to etch the complete high resolution pattern into the target layer 4. Once this has been done, the residual hardmask 6 is removed, for example using etching, to yield the final structure shown in FIG. 1j. 